1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus having an intermediate transferring member and electrifying (charging) means for electrifying developer remaining on the intermediate transferring member after secondary transferring.
2. Related Background Art
FIG. 10 shows an example of a conventional color image forming apparatus of intermediate transferring belt type using an intermediate transferring belt as an intermediate transferring member. A photosensitive drum 1 as an electrostatic latent image bearing member is constituted by applying organic photoconductor (OPC) or photoconductor formed from A—Si, CdS or Se onto an outer peripheral surface of a metal core, which drum forms a first image bearing member.
In an exposure position 3a on a surface of the photosensitive drum 1, a latent image is formed by an exposing apparatus 3. A contact portion between the photosensitive drum 1 and an intermediate transferring belt 6a defines a position (primary transferring portion) T1 for performing primary transferring. A distance from the exposure position 3a of the photosensitive drum 1 to the primary transferring portion T1 in a counter-clockwise direction in FIG. 10 is defined as a distance L1, and a position spaced apart, by L1, from the primary transferring portion T1 on the intermediate transferring belt 6a in an upstream direction is defined as an S portion.
The intermediate transferring belt 6a as a second image bearing member is formed from rubber such as EPDM, NBR, urethane or silicone rubber, or resin such as PI, PC, PVDF, ETFE, PET, PC/PAT, ETFE/PC, ETFE/PAT or PC/PAT and is mounted, in tension, around three rollers, i.e., a drive roller 6b, a tension roller 6c and a secondary transferring counter roller 6d so that the belt is driven in a driven direction shown by the arrow C3 with respect to the photosensitive drum 1 rotating in a direction C1 by rotating the drive roller 6b by means of a motor (not shown).
Further, a primary transferring roller 7b constituted by a shaft and a conductive sponge layer thereon is contacted with the photosensitive drum 1 with the interposition of the intermediate transferring belt 6a in the primary transferring portion T1.
Further, at one circumferential portion of the intermediate transferring belt 6a, i.e., at an end transverse to a circumferential direction, that is, an end of the belt 6a, there is provided a position detecting member 5a which can be distinguished from the surface of the intermediate transferring belt 6a by means of an optical sensor 5, so that a timing when the position detecting member 5a passes through a detecting portion 5b due to rotation of the intermediate transferring belt 6a is detected by the optical sensor (referred to as “TOP detection” hereinafter) and a rotating condition of the intermediate transferring belt 6a can be recognized by counting a time from the detecting timing within a main body of the apparatus.
Alternating voltage obtained by overlapping alternating voltage with DC voltage from an alternating voltage power supply 10a is applied to an electrifying roller 10b provided around the intermediate transferring belt 6a and disposed between a position (secondary transferring portion) T2 for performing the secondary transferring and the primary transferring portion T1. Incidentally, the electrifying roller 10b has an abutment/separation mechanism (not shown) with respect to the intermediate transferring belt 6a and can abut against the belt at a desired timing. Further, a counter electrode 10c for grounding to enhance electrifying efficiency is provided on a back surface of a portion of the intermediate transferring belt 6a against which the electrifying roller 10b abuts.
Now, an image forming operation will be described with reference to FIG. 10.
The photosensitive drum 1 and the intermediate transferring belt 6a are rotated in normal directions shown by the arrows C1 and C3, respectively, at the same speed (referred to as “process speed” hereinafter) V of 118.0 mm/s. The photosensitive drum 1 having a circumferential length of 147.5 mm is rotated by one revolution at 147.5/V=1.25 s and the intermediate belt 6a having a circumferential length of 442.5 mm is rotated by one revolution at 442.5/V=3.75 s.
The photosensitive drum 1 is driven in the direction C1 by drive means (not shown) and is uniformly electrified with predetermined potential by an electrifying roller 2. When the TOP detection of the rotating intermediate transferring belt 6a is performed by the optical sensor 5, the exposure is started by the exposing apparatus 3, with the result that a light signal corresponding to a yellow pattern is scanned on the uniformly electrified photosensitive drum 1, thereby forming a latent image.
After the latent image formation is started, when the photosensitive drum 1 is further rotated in the direction C1, a support 4 is rotated in a direction shown by the arrow C2 so that a developing apparatus 4a containing yellow toner among developing apparatuses 4a, 4b, 4c and 4d supported by the support 4 is opposed to the photosensitive drum 1, with the result that latent image formed on the photosensitive drum 1 is visualized by the selected developing apparatus 4a to form a developer image (toner image). Here, upon the development, the toner has negative polarity.
When the photosensitive drum 1 is further rotated in the direction C1 to reach the primary transferring portion T1, the developed toner image is primarily transferred onto the intermediate transferring belt 6a by applying primary transferring bias having positive polarity opposed to the negative polarity toner upon the development from a high voltage power supply 7a to the primary transferring roller 7b, using the metal core of the photosensitive drum 1 as a counter electrode.
After the transferring, residual toner remaining on the photosensitive drum 1 is cleaned or removed by a cleaner 13.
After a time (L1/V) is elapsed from the start of the exposure, a point on the photosensitive drum 1 from where the writing of an image is started and a point 6S on the intermediate transferring belt 6a which has passed through the S portion upon initiation of the exposure coincide with each other at the position T1. That is to say, the image is formed from its leading end on the intermediate transferring belt 6a in a counter-clockwise direction.
When the development of the yellow toner image is finished, the developing apparatus is switched to a new developing apparatus. In a printing sequence according to the present invention, by using a timing design for forming an A4 image, i.e. an image of 297 mm, the sequence can be utilized with respect to all sizes smaller than A4, thereby making the process in common. (Hereinafter, an A4 mode will be explained.) When a time (297 mm/V) is elapsed after the leading edge of the image reached the developing portion, the support 4 is rotated in the counter-clockwise direction, with the result that the developing apparatus 4b containing magenta toner is positioned to be opposed to the photosensitive drum 1.
Further, when the next TOP detection is performed, similar operations are repeated, so that the developing and transferring operations are performed with respect to a magenta color, a cyan color and a black color, with the result that plural color toner images are formed on the intermediate transferring belt 6a in a superimposed fashion.
In this case, since all of four color images are transferred so that the leading edge of each image coincides with the point 6S, the four color toner images are registered with each other.
When the four color toner images are transferred to the intermediate transferring belt 6a in the superimposed fashion, a transferring material P is conveyed from a registration roller pair R in synchronism with the movement of the intermediate transferring belt 6a and a secondary transferring roller 9 having the similar construction to that of the primary transferring roller 7b abuts against the intermediate transferring belt 6b with the interposition of the transferring material P at the secondary transferring portion T2, and, by applying secondary transferring bias having positive polarity from a high voltage power supply (not shown) by utilizing the secondary transferring counter roller 6d supporting the intermediate transferring belt 6a as a counter electrode, the four color toner images on the intermediate transferring belt 6a are secondarily transferred onto the transferring material P collectively.
The transferring material P to which the four color toner images were transferred is sent to a conventional fixing apparatus 11 of heat and pressure type, where the toner images are fused and fixed, thereby forming a color image.
Now, electrifying and recovering of secondary transferring residual developer (secondary transferring residual toner) remaining on the intermediate transferring belt after the secondary transferring will be explained with reference to FIG. 11 on the basis of a normal page interval process or step. Incidentally, in FIG. 11, developments for first to fourth colors are designated by Dv1 to Dv4, primary transferring operations for first to fourth colors are designated by Tr1 to Tr4, secondary transferring is designated by Tr2, electrifying (charging) of the secondary transferring residual toner is designated by Ch2, and recovering of such residual toner at the primary transferring portion T1 is designated by RET.
Charges of positive polarity are uniformly applied to the secondary transferring residual toner not transferred to the transfer material P and remaining on the intermediate transferring belt 6a after the secondary transferring to the transferring material P by an electrifying roller 10b to which alternating voltage of positive polarity obtained by overlapping alternating voltage with DC voltage is applied from an alternating voltage power supply 10a. 
Then, the secondary transferring residual toner electrified with positive polarity by the electrifying roller 10b reaches the primary transferring portion T1 due to the rotation of the intermediate transferring belt 6a and is removed from the intermediate transferring belt 6a by electrostatically transferring the toner to the photosensitive drum 1 at the same time as primary transferring of a first color of a next page.
The secondary transferring residual toner transferred to the photosensitive drum 1 is recovered into a photosensitive drum cleaner 13. In this way, removal of the secondary transferring residual toner on the intermediate transferring belt 6a is completed.
Incidentally, various operations, i.e. the printing sequence for forming the image includes at least two processes, i.e. a continuous image forming process for performing the above-mentioned image formation continuously, and a process effected after the continuous image forming process, for cleaning the second image bearing member to remove the secondary transferring residual toner in the last image formation in the continuous image forming process, toner naturally (mechanically without voltage) discharged from the secondary transferring residual toner electrifying roller 10b and toner flying within the apparatus.
On the other hand, whenever the image formation is performed, since adhering (or sticking) developer (adhering (or sticking) toner) having negative polarity which was not electrified when the secondary transferring residual toner is electrified with positive polarity is adhered to the electrifying roller 10b, if images are formed continuously through plural pages, the adhering toner will be accumulated.
Further, since the adhering toner gradually worsens electrifying performance of the electrifying roller 10b to cause poor cleaning of the intermediate transferring belt 6a due to poor electrifying, it is necessary to effect the cleaning for removing the adhering toner on the electrifying roller 10b. 
In the past, a process for removing the toner adhered to the electrifying roller 10b was performed in the post-rotation process of the printing sequence. Now, an example of such a process will be described.
The removal of the adhering toner is performed by discharging the adhering toner from the electrifying roller 10b onto the intermediate transferring belt 6a by applying bias having negative polarity to the electrifying roller 10b. When the discharged toner reaches the primary transferring portion T1, the bias to be applied to the primary transferring roller 7b is switched to negative polarity, thereby transferring the toner onto the photosensitive drum 1 by an electrostatic repelling force. Lastly, the toner is recovered by the photosensitive drum cleaner 13. In this way, the removal is completed. Timings of such operations will be explained with reference to FIG. 12.
After the secondary transferring in the image formation of the last page among a predetermined number of pages (page number) in the continuous image forming process is finished, the post-rotation process is started. In the post-rotation process, the secondary transferring residual toner (of the plural color toner images) remaining on the intermediate transferring belt 6a after the secondary transferring regarding the last page in the continuous image forming process is electrified with positive polarity and is recovered at the primary transferring portion T1. Further, the excessive toner adhered to the second image bearing member such as the toner naturally (mechanically without voltage) discharged from the secondary transferring residual toner electrifying roller 10b and the toner flying within the apparatus is also recovered at the primary transferring portion T1 simultaneously.
In the post-rotation process, the bias to be applied to the primary transferring roller 7b at any timing is switched to negative polarity so that the recovering of the toner having negative polarity and discharged from the electrifying roller 10b onto the intermediate transferring belt 6a is permitted.
Further, in coincidence with the timing for applying the negative polarity bias, the bias being applied from the alternating voltage power supply 10a to the electrifying roller 10b is switched to alternating voltage having negative polarity, thereby starting the discharging of the adhering toner.
That is to say, at the timing for switching the bias to be applied to the electrifying roller 10b to the negative polarity, when the portion of the intermediate transferring belt 6a which has passed through the electrifying roller 10b reaches the primary transferring portion T1, the bias to be applied to the primary transferring roller 7b is also switched to the negative polarity. After the adhering toner is discharged for a predetermined time and is recovered at the primary transferring portion T1, the bias being applied to the electrifying roller 10b is turned OFF, and, when the portion passed through the electrifying roller 10b at that time reaches the primary transferring portion T1, the bias to be applied to the primary transferring roller 7b is also turned OFF. Such operations are the post-rotation process.
By doing so, the discharged toner is electrostatically transferred onto the photosensitive drum 1 and thus is removed from the intermediate transferring belt 6a. The discharged toner transferred to the photosensitive drum 1 is recovered by the photosensitive drum cleaner 13. In this way, the removal of the toner adhered to the electrifying roller 10b is completed.
However, if the images for a large number of pages are formed in one printing sequence, the toner adhered to the electrifying means is accumulated excessively and be dropped to contaminate the interior of the apparatus and/or the transferring material and, in the removal process for removing the adhering toner, the adhering toner transferred to the intermediate transferring belt may not be recovered by the photosensitive drum adequately to contaminate the intermediate transferring belt and/or to cause poor image.
Such inconveniences are easily generated particularly when a printing ratio of the image in which the secondary transferring residual toner increased is high and, under high temperature/high humidity and low temperature/low humidity environments where the secondary transferring ability is worsened.